This invention relates to a tilt device for controlling the optical axis of an optical head in an optical disc playback device for playing back optical discs such as a Laser Vision Disc (LV) and Compact Disc (CD) in a perpendicular relation to the surface of an optical disc and, more particularly, to a tilt device of such type capable of performing servo-control so as to maintain distance between the optical pickup and the surface of an optical disc constant.
In an optical disc playback device, an optical head including an optical pickup is used for picking up signals recorded on the disc surface of the optical disc. Picking up of signals is usually effected by displacing the optical pickup in the radial direction of the optical disc with the optical axis of the optical pickup being maintained perpendicular to the disc surface regardless of position of the optical pickup.
Optical discs, however, often have flexion originally caused by actions of heat and gravity or are bent downwardly in their outer peripheral portion due to their weight when they are loaded in the optical disc playback device. The amount of flexion is particularly large when the optical disc is of a large diameter such as a video disc. When such flexion occurs, the optical axis of the optical pickup tend becomes inclined and deviated from the perpendicular relation to the disc surface.
A tilt control therefore has been practiced for controlling the angle of the optical pickup so as to maintain the optical axis constantly perpendicular to the disc surface.
An example of a prior art tilt control device is shown in FIGS. 7 and 8. In this prior art device, there is provided a pivoting axis 3 in parallel with an optical disc D and normally to the radial direction of the optical disc D in an intermediate portion between the center and the outer periphery of the optical disc D on a base frame 2 on which a drive motor 1 for rotating the optical disc D is mounted. A subframe 4 is pivotably supported on this pivoting axis 3.
A pair of feed shafts 6 along which the optical pickup 5 is guided in sliding movement in the radial direction of the optical disc D are fixedly secured at both ends thereof to this subframe 4. A tilt feed screw 7 which is in threaded engagement with the subframe 4 at its end portion nearer to the outer periphery of the optical disc D is rotated by a drive motor 8 to change the distance between the subframe 4 and a base frame 2 and thereby pivot the subframe 4 about the pivoting axis 3.
For controlling this tilt mechanism, a tilt angle sensor 9 is provided adjacent to the optical pickup 5 and the tilt drive motor 8 is controlled in response to a detection signal produced by the tilt angle sensor 9 to apply servo for maintaining the optical axis of the optical pickup constant.
There is another prior art tilt mechanism as shown in FIG. 9.
In this tilt mechanism, a feed shaft 6 which is parallel with an optical disc D and disposed in the radial direction of the optical disc D is fixedly secured at both ends thereof on a base frame 2 on which a disc drive motor 1 is mounted. A subframe 4 which is reciprocable along the feed shaft 6 is provided.
There is provided a shaft 13 which is pivotable about a pivoting axis 3 which is parallel with the optical disc D and normal to the radial direction of the optical disc D and an optical pickup 5 is mounted on this shaft 13.
A tilt feed screw (not shown) which is in threaded engagement with an end portion of the shaft 13 nearer to the outer periphery of the optical disc D is rotated by a tilt drive motor (not shown) to change the distance between the subframe 4 and the shaft 13 and thereby change the angle of the optical pickup 5 together with the shaft 13 by pivoting the shaft 13 about the pivoting axis 3.
For controlling this tilt mechanism, the same servo device as that shown in FIG. 7 is employed. Namely, the tilt drive motor is controlled in response to a detection signal produced by the tilt angle sensor 9 provided adjacent to the optical pickup 5 to incline the shaft 13 so that the optical axis of the optical pickup 5 is maintained at a constant angle.
In both the tilt mechanism shown in FIG. 7 in which the feed shafts themselves are inclined and the tilt mechanism shown in FIG. 9 in which the shaft 13 which is separate from the feed shaft 6 is inclined, the servo control of the tilt mechanism is performed in such a manner that the angle of the optical axis of the optical pickup is maintained constant in response to a value detected by the tilt angle sensor 9. If, in these prior art tilt mechanisms, however, the optical axis of the optical pickup is made perpendicular to the disc surface of the optical disc D regardless of the position of the optical pickup in the radial direction of the optical disc D, distance between the optical disc D and the optical pickup 5 changes from L1 to L2 between a position nearer to the center of the optical disc D and a position nearer to the outer periphery thereof as shown in FIGS. 9 and 10, when there is a large flexion in the optical disc D.
For this reason, a focus stroke in a focus servo system must be made large in order to focus laser beam on the disc surface and this is extremely inconvenient in the design of the optical disc playback device.
It is, therefore, the first object of the invention to provide a tilt device for an optical head capable of maintaining the optical axis of an optical pickup approximately perpendicular to a bent surface of an optical disc with a simple construction and with a relatively small focus stroke in the focus servo system.
Further, if flexion occurring in an optical disc D is closely observed, an optical disc D having such flexion has a curved surface which bends downwardly toward the outer periphery of the optical disc D. In the prior art tilt mechanism in which the linear feed shaft 6 secured at its ends on the subframe 4 is tilted with the subframe 4, the problem that the linear feed shaft 6 cannot cope with the arcuate flexion of the optical disc D becomes more serious as the optical pickup 5 proceeds to the outer periphery of the optical disc D.
Besides, since the pivoting axis 3 about which the subframe 4 of the tilt mechanism shown in FIG. 7 is disposed intermediate between the center and the outer periphery of the optical disc D so as to minimize the range of pivoting of the subframe 4, the mounting position of the pivoting axis 3 is spaced away from the center spindle. As a result, it becomes necessary to determine this mounting position accurately because otherwise the optical pickup 5 would not be able to move along the feed shaft 6 in exact radial direction of the optical disc D so that alignment after assembly of the device would become necessary.
Further, the prior art device is complex in its structure because the tilt mechanism requires the subframe 4 and this subframe 4 is mounted on the base frame 2 through the pivoting axis 3. In addition, the prior art device requires space in vertical direction, particularly in the vicinity of the disc drive motor 1, for allowing the tilt operation.
It is, therefore, the second object of the invention to provide a tilt device which has eliminated these disadvantages in the prior art tilt device.
In the prior art tilt devices shown in FIGS. 7 and 9, the servo control is performed in such a manner that the angle of the optical axis of the optical pickup 5 is made perpendicular in accordance with a detection signal provided by the tilt angle sensor 9 which is provided separately from the optical pickup 5. There has arisen the problem in these prior art devices that the center of the tilt angle sensor 9 is not in agreement with the center of the optical pickup 5 with resulting occurrence of a detection error.
Besides, in the prior art tilt devices using the tilt angle sensor 9 which is separate from the optical pickup 5, the angle of the optical disc D cannot be detected in an area between the optical pickup 5 and the tilt angle sensor 9 when the optical pickup 5 is positioned at the nearest position to the center of the optical disc D so that tilt control over the entire playback range cannot be made.
Furthermore, detection of the angle of the optical disc D by the tilt angle sensor 9 depends upon laser beam reflected from the surface of the optical disc D so that, when there is dust or scar on the surface of the optical disc D, there occurs an error in the detected signal.
It is, therefore, the third object of the invention to provide a tilt device capable of performing tilt control so as to make the optical axis of an optical pickup approximately perpendicular to the disc surface of the optical disc over the entire playback range of the optical disc without employing a tilt angle sensor.